Julian Assange's supporters have pleaded in court to keep £140,000 in bail money that is in jeopardy because of the WikiLeaks founder's decision to seek political asylum in the Ecuadorean embassy in London.

Vaughan Smith, the former British army captain who hosted Assange at his Norfolk home while he was on bail throughout 2011, told Westminster magistrates court that Assange's nine sureties were powerless over the man whose compliance with the legal process they had guaranteed back in 2010. He said Assange's decision to evade a European arrest warrant had become an "unprecedented" matter of diplomatic and inter-governmental concern, which they could not reasonably influence.

Smith spoke on behalf of sureties who include Philip Knightly, a veteran Australian investigative journalist who exposed the British traitor Kim Philby as a Russian spy, and Sir John Sulston, a Nobel prize-winning biologist. Smith told the court that a group of them visited Assange on Monday and concluded "the sureties do not have the power to meaningfully intervene in this matter. This has become a matter between the Ecuadorean, British, Swedish, US and Australian governments."

The supporters provided between £5,000 and £20,000 each to secure Assange's freedom when he was arrested in December 2010 in connection with allegations of rape and sexual assault relating to relationships with two women in Stockholm. Assange broke his bail conditions in June this year when he took refuge in the Ecuadorean embassy in Knightsbridge after he lost a supreme court challenge to the validity of the European arrest warrant that demanded his return to Sweden for questioning.

It is understood that a separate group of Assange supporters, thought to include the film-maker Ken Loach, the writer and campaigner Jemima Khan, the journalist John Pilger and the magazine publisher Felix Dennis have already forfeited bail cash worth £200,000 following a court order earlier this year.

"We never envisaged when we became sureties that the matter would become a diplomatic argument and it is clear that this needs to be resolved at diplomatic level," said Smith in the short hearing in front of the chief magistrate, Howard Riddle, which was attended by four other sureties: Knightly, Prof Patricia David, a retired professor of education, Lady Caroline Evans, the wife of the former Labour minister Lord Evans, and Tracy Worcester, a model turned environmental activist.

Smith told the court that the sureties could not ignore the perceived risk that if Assange is extradited to Sweden he could end up in a US prison "under unjust conditions".

To publicly urge him to leave the embassy would "render us mercenary and contemptible individuals of great weakness of character", Smith said.

He urged the court to take into account the "huge amount of effort over an unexpectedly long period" the sureties had put in to support the legal process. He drew attention to his own efforts and those of his family, who as well as standing £20,000 in bail money hosted Assange for 13 months. He also highlighted the efforts of Sarah Saunders, a WikiLeaks activist and surety, who hosted Assange at her Kent home from where he had to report each day to a police station and adhere to a 10pm-to-8am curfew.

"We request that sureties in this case be treated gracefully, in a manner that reflects the impossible position that we are in," he said.

"In this unique, this quite exceptional case, complying with what this court seems to expect from us, to all publicly urge Mr Assange to abandon the sanctuary that he has found in the Ecuadorean embassy, would see us acting against a man whom we and others judge to have understandable fears about his ultimate treatment in the United States if he abandons his asylum."

The chief magistrate said he would take several days to consider the submission and other evidence before making a ruling. In an earlier hearing in September, he had given the nine sureties a month to persuade Assange to surrender himself to police if they wanted their money back, saying he was not persuaded that they were not "using every effort, publicly and privately, to persuade Mr Assange to surrender himself to UK authorities".

Sweden said in August that any suggestion that Assange could be extradited to America, where he is wanted for his role in leaking hundreds of thousands of classified US military and diplomatic papers, was "hypothetical speculation that shifts the focus away from what this case is actually about". Assange has claimed he could face the death penalty in the US.

Karin Hoglund, deputy ambassador at the Swedish embassy in Washington DC, said the issue was "exclusively a question of surrender to Sweden under a European arrest warrant on Mr Assange, who is under criminal investigation in Sweden suspected of sexual offences against two Swedish women".

She said: "Sweden is not allowed to extradite a person at risk of capital punishment … The extradition act also includes grounds for refusal of extradition, such as political or military offences and situations in which the person who is extradited is at risk of persecution."